don't accept invalid certs even in tests

[nothingmuch]

This is a minor cleanup change from the WIP self signed certificates that can be merged independently.

Turns out we're actually not (no longer?) relying on invalid certificates because the with_cert functionality just asserts that the self signed certificate, which is its own root, is valid.

[coveralls]

Pull Request Test Coverage Report for Build 13466665336

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.008%) to 79.045%

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Totals
Change from base Build 13466524337:	-0.008%
Covered Lines:	4040
Relevant Lines:	5111

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💛 - Coveralls

[DanGould]

ACK edb595e

This is a more restrictive change, so since tests pass it looks good to me.